The No. 2 reported sexually transmitted disease: gonorrhea. While gonorrhea rates remain near the record low, there are ominous signs the disease known as "the clap" may make a comeback. Gonorrhea may be becoming resistant to the antibiotics used to treat the disease.

But it's chlamydia that steals top billing in today's 187-page CDC report on the U.S. trends of STDs (sexually transmitted diseases). The report contains some alarming numbers:

Overall, 1.3 million chlamydia cases were reported to the CDC in 2010 -- the most ever.

There are 426 reported cases of chlamydia per 100,000 Americans. But rates range from 186 cases per 100,000 people in New Hampshire to a whopping 862 cases per 100,000 in Alaska.

About 8% of U.S. women ages 15 to 24 test positive for chlamydia at family planning clinics.

The rate of chlamydia among African-Americans is over eight times higher than the rate among whites. However, among both African-Americans and whites chlamydia rates are increasing at about the same speed.

Perhaps the most sobering thing about the increase in chlamydia is that rising rates don't necessarily mean there's an actual increase in the number of people with chlamydia.

Instead, better screening -- including a urine test -- means we're only now beginning to realize how big a problem chlamydia has been all along.

For example, only 25% of young women were screened for chlamydia in 2000. That went up to 48% in 2010. That's a lot more, but we're far from finding all the women and men who need treatment.

Chlamydia is treatable with antibiotics. Sadly, many people don't know they have a chlamydia infection. Complications are uncommon in men, but untreated chlamydia causes pelvic inflammatory disease (PID) in 10% to 15% of women. It can also cause permanent damage to a woman's reproductive organs, making her sterile.

Rising gonorrhea drug resistance tempers the good news that gonorrhea rates are near their historic low. In a news release, the CDC notes that "the disease may become resistant to the only available treatment option."

If there's any good news in the CDC report, it is that the syphilis rate went down for the first time in 10 years. But there's bad news on syphilis, too. The rate among African-American men has gone up 134% over the last five years, driven mainly by an increase among young African-American men who have sex with other men.